A. Field of the Invention
The present invention relates generally to detection systems, and more particularly, to a detection system for accurately determining the directions and ranges of lightning discharges to ground.
B. Description of the Prior Art
In many applications, it is often necessary to determine accurate locations of lightning discharges to ground. For example, accurate knowledge of lightning discharges aids in the precise location of forest fires and power system damage, and is useful in providing an indication of the path of an approaching thunderstorm so that recreational areas such as golf courses and swimming pools in the path of the storm may be safely evacuated.
Several systems for locating lightning discharges or thunderstorms are known. One such system employs radar for detecting thunderstorm activity, while another system utilizes a magnetic cathode ray direction finder employing a pair of orthogonal loop antennas tuned to a VLF frequency, typically 10 KHz, for detecting the horizontal magnetic field produced by lightning. The outputs of the orthogonal antennas are simultaneously applied to the X and Y inputs of the X-Y oscilloscope to generate a vector display indicating the direction of the lightning. The intersection of the vectors generated by two geographically spaced direction finders indicates the location of the discharge.
While these systems do provide a way to detect the presence of thunderstorm activity, radar systems do not detect lightning discharges directly, but rather detect precipitation and the presence of clouds. However, the presence of precipitation or clouds is not, by itself, an accurate indication of the presence of lightning, because many large clouds may produce little or no lightning, while a single small cloud may produce severe lightning discharges to ground.
Magnetic cathode ray direction finders detect lightning discharges directly, but the accuracy of conventional cathode ray direction finders is poor at distances of less than about 200 km because antennas sense not only the magnetic field from the vertical channel, but also the fields produced by horizontal channel sections and by ionospheric reflections. Also, these systems are subject to site errors due to their narrow bandwidths. These effects result in bearing errors in excess of 20.degree. at distances of about 200 km, with the errors growing substantially larger for distances of less than 200 km. Furthermore, the prior art systems are unable readily to discriminate between ground strokes and intracloud discharges.